An envelope for a flat battery element is known from patent application EP 0 176 597 of Mar. 29, 1984 (SEALED BATTERY). This document describes a flat battery element having electrodes and an electrolyte enclosed in an envelope. The electrodes have contact zones for their connection with external contact pins, and to this end extend through an impervious sealing zone of the pocket and pass beyond the exterior of the envelope. The envelope is enclosed in a housing constituted by two shells which are sealed at their edges and thus ensure a pressure on the walls of the envelope. One of the shells of the housing is provided with input/output pins arranged in a part which is not under pressure. The connection between said pins and the contact zones of the electrodes is made directly outside the envelope and also inside the housing.